1. Field of the Invention
The present invention relates to a cable modem tuner. More specifically, the present invention relates to a cable modem tuner suitable for outputting an intermediate frequency signal to a QAM demodulating circuit.
2. Description of the Background Art
In a cable television system (hereinafter referred to as CATV), introduction of HFC (Hybrid Fiber/Coax) has been in progress, in which a coaxial cable is kept as a subscriber's drop wire and the main network is implemented by optical fibers. This system attempts to provide broad-band data communication service of several Mbits/sec at home. Utilizing this system, it is possible to realize high speed data line having the transmission rate of 30 Mbits/sec with the bandwidth of 6 MHz using 64 QAM (Quadrature Amplitude Modulation), which may not be called the state of the art any more. The cable modem is used in this system, and realizes high speed data communication of 4 Mbits/sec to 27 Mbits/sec, utilizing an unused channel of CATV. The cable modem tuner is used for a cable modem in such a CATV system, and after the received CATV signal is subjected to frequency conversion, it serves to take out the signal as an intermediate frequency signal.
FIG. 10 is a block diagram representing a configuration of a conventional cable modem tuner 1.
As for the CATV signals, an up signal transmitted to a station has the frequency of 5 MHz to 42 MHz, and a down signal transmitted from the station to the cable modem tuner has the frequency of 54 MHz to 860 MHz, and transmitted to a cable network through an input terminal 2 of the tuner. The up signal transmitted from the cable modem is received by a data receiver of the CATV station (system operator), and enters a computer of a center.
Referring to FIG. 10, the cable modem tuner 1 includes a CATV signal input terminal 2 receiving the CATV signal as an input, a data terminal 3 receiving a data signal from a QPSK transmitter as an input, and an upstream circuit (LPF) 4 provided between data terminal 3 and CATV signal input terminal 2. In the cable modem, a data signal subjected to quadrature phase shift keying (QPSK) from a QPSK transmitter, for example, is input to data terminal 3 as the up signal. The data signal is transmitted through upstream circuit 4 to the CATV station.
The down signal input through input terminal 2 is divided into a UHF band (hereinafter also referred to as a B3 band) receiving the frequency of 470 to 860 MHz, a VHF-High band (hereinafter also referred to as B2 band) receiving the frequency of 170 to 470 MHz and a VHF-Low band (hereinafter also referred to as B1 band) receiving the frequency of 54 to 170 MHz, and processed by receiving circuits provided for respective bands. Band ranges are not limited to those specified above.
The cable modem tuner 1 further includes a high pass filter (HPF) 5 having an attenuation range of 5 to 46 MHz and a pass band of not lower than 54 MHz, and input switching circuits 6 and 7 for allocating the signals passed through the high pass filter 5 to circuits corresponding to respective bands.
The down signal is passed through high pass filter 5, the band is switched by the input switching circuits 6 and 7, and supplied to the circuitry corresponding to any of the aforementioned bands B1 to B3.
Cable modem tuner 1 further includes high frequency amplification input tuning circuits 8, 9 and 10 provided corresponding to respective bands B1 to B3; high frequency amplification AGC circuits 11 and 12 provided corresponding to the UHF band and VHF band, respectively; high frequency amplification output tuning circuits 15, 16 and 17 provided corresponding to respective bands B1 to B3; a mixer circuit 18 and a local oscillation circuit 19 provided corresponding to the UHF band; a mixer circuit 20 and a local oscillation circuit 21 provided corresponding to the VHF band; and an intermediate frequency amplifying circuit 22 for amplifying, in the intermediate frequency band, outputs from mixer circuits 18 and 20.
The high frequency amplification input tuning circuits, the high frequency amplification AGC circuits, the high frequency amplification output tuning circuits, the mixer circuits and the local oscillation circuits provided corresponding to respective bands are adapted such that dependent on the received channel, circuits corresponding to the received band are made operative, while the circuits corresponding to other bands are made inoperative. For example, when a UHF channel is received, the high frequency amplification input tuning circuit 8, the high frequency amplification AGC circuit 11, high frequency amplification output tuning circuit 15, mixer circuit 18 and local oscillation circuit 19 for the UHF band are set to the operative state, while high frequency amplification input tuning circuits 9 and 10, high frequency amplification AGC circuit 12, high frequency amplification output tuning circuits 16 and 17, mixer circuit 20 and local oscillation circuit 21 for the VHF-High and VHF-Low bands are set to inoperative state, and stop their operation.
The CATV signal input to the input terminal 2 is passed through high pass filter 5 as described above, and enters input switching circuits 6 and 7, where band switching takes place. The output therefrom is fed to high frequency amplification input tuning circuit 8, 9 or 10, where channel selection takes place. After channel selection, the signal is input to AGC terminal 24, amplified to a prescribed level by high frequency amplification circuit 11 or 12 based on the AGC voltage applied to resistance 13 or 14, supplied to high frequency output tuning circuit 15, 16 or 17, where the received signal is extracted.
Thereafter, the selected received signal is subjected to frequency conversion by mixer circuit 18, 20 and local oscillation circuit 19, 21 to an intermediate frequency (hereinafter also referred to as IF), and amplified by intermediate frequency amplification circuit 22.
The intermediate frequency signal hereinafter referred to as an IF signal) amplified by intermediate frequency amplifying circuit 22 is output from output terminal 23.
In this manner, in the conventional cable modem tuner 1, a received CATV signal is selected in accordance with the reception channel, and the signal after channel selection is subjected to frequency conversion and output as an IF signal from output terminal 23.
Handling of a QAM signal, which is a digital signal, and transmitting the IF signal output from output terminal 23 to a QAM demodulating circuit, not shown, for QAM demodulation by using such a cable modem tuner 1 has the following various problems.
(1) First, dependent on the type of QAM demodulating IC used as the QAM demodulating circuit, IF signal of a different frequency band becomes necessary. In the following description, of the IF signals, those output from the conventional cable modem tuner are described as having the frequency band of High-IF, and the frequency range generally not higher than 10 MHz and lower than the High-IF will be referred to as Low-IF. At present, QAM demodulating ICs include ICs for receiving Low-IF QAM signals and ICs for receiving High-IF QAM signals. These result from the limitation imposed by the performance of analog/digital converter used in the QAM demodulating ICs. Accordingly, in order to attain a frequency range that can be received by the QAM demodulating IC connected in the succeeding stage, two different types of cable modem tuners have been necessary, or a frequency converting circuit positioned between the cable modem tuner and the QAM demodulating IC has been necessary.
(2) It is specified in DOCSIS (cable modem specification in North America) that the maximum output level of the transmitted upstream signal must be constant and +58 dBmV, and a signal level satisfying this specification is necessary at the input end of the tuner. The input level of the conventional cable modem tuner has been not up to this level.
(3) According to DOCSIS, it is required that the upstream transmission signal can be variably controlled 1 dB by 1 dB, from +58 to +6 dBV. Conventionally, such a function was not required.
(4) According to DOCSIS specification, the high frequency level of the transmission signal must be at least −50 dBmV, and in the example shown in FIG. 10, it is necessary to improve significantly from the conventional level.
(5) A further problem is that measures against digital noise are necessary. As the QAM demodulating IC requires high input signal level, an amplifier having a high gain is necessary. Therefore, when the overall system is configured, the clock noise and the bus noise of a CPU (Central Processing Unit) also come to have high levels. As it is a common practice to mount the QAM demodulating IC, the CPU and the cable modem tuner on one board, influence of such noises would be significant.
Though a cable modem tuner is shown in FIG. 10 described above, a CATV tuner referred to as a digital set top box (hereinafter referred to as STB) has come to be used. In the cable modem, the down data signal transmitted from the CATV station is displayed on a television monitor, while in the STB, a QPSK modulated down data signal transmitted from the CATV station is branched from the tuner portion, and processed by CPU to be output to a personal computer.
Accordingly, in the cable modem, an unused channel of the CATV in 54 MHz to 860 MHz band is used for transmitting the down data signal as described above, whereas in the STB, the frequency band of 70 MHz to 130 MHz is used.
In the STB, there is a branching circuit provided for branching the down data signal on the output side of the HPF shown in FIG. 10, and the branched down data signal is output to an OOB (Out Of Band) terminal. The OOB terminal provides the branched data to CPU.
In the STB also, as for the CATV signals, the up signal has the frequency of 5 MHz to 42 MHz and the down signal has the frequency of 54 MHz to 860 MHz, and connected to the cable network through input terminal 2. The up signal transmitted from the STB is received by the data receiver at the CATV station, and input to a computer of the center.
In the STB, the data signal subjected to QPSK from QPSK transmitter (not shown) is introduced to the data terminal as the up signal. The data signal is fed to the STB by the computer at the center through the CATV circuit, processed by the CPU (not shown) in the STB, and applied to the QPSK modulator. Except for these points, the operation is similar to that in the cable modem tuner shown in FIG. 10, and hence, the STB also has the same problems as the cable modem tuner described above.